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eVGA GeForce 7950 GX2: One Card, Double the Fun

Posted by Parth Barot on June 21, 2006

By Jason Cross

It’s hard to believe that almost two years have passed since Nvidia first introduced SLI—or should we say, re-introduced it. The idea that you can use two consumer graphics cards in parallel to nearly double graphics performance goes back to the days of 3dfx, where two PCI graphics cards would draw alternate lines in “scan line interleave” mode. The proliferation of AGP graphics pretty much killed that concept, but the flexibility of PCI Express brought it back. As PCIe graphics started to take hold, Nvidia introduced its own SLI—Scalable Link Interface—together with an nForce 4 motherboard chipset that sported two graphics card slots.

And the graphics world would never be the same. SLI was a curiosity in the 3dfx days, but dual graphics configurations now look like a serious point of contention in a market that has narrowed to two industry giants while growing all the more competitive. Systems using two graphics cards in parallel are still not all that common, but the capability is there in millions of PCs across a broad price spectrum. nForce motherboards to support SLI are available at very affordable prices, and you don’t have to pay through the nose to get a board compatible with Crossfire—ATI’s dual-graphics answer to SLI—either.

While ATI has made some good strides with Crossfire, coming late to the dual-graphics game and trying to play catch up, Nvidia has remained one step ahead. It seems like Nvidia always has a new dual-graphics trick up its sleeve, whether it’s cheaper SLI motherboards, cheaper SLI-capable graphics cards, or drivers that enable new features in SLI mode. Their latest trick is maybe their best one yet—SLI on a single graphics card (sort of). The GeForce 7950 GX2 marries two 7900 GT-class GPUs, with two circuit boards, into a single double-wide card. Let’s take a look at the new two-headed monster.

Single Slot SLI

SLI is nifty, but the big problem with it is, well, it’s SLI. It requires an SLI motherboard with an nForce chipset. We tend to like Nvidia’s motherboard chipsets, but it’s still a limiting factor. It requires two graphics cards stuffed into two slots, which doesn’t fit well in every case (particularly small-form-factor cases). Let’s not forget that most medium to high-end graphics cards require a power connector, so you’ll need two of those, and sometimes one heck of a beefy power supply. These problems aren’t unique to SLI, of course. ATI shares these dual-graphics challenges.

What if you could basically sandwich two single-slot graphics cards together into one double-wide card that uses just one slot? Essentially, that is what Nvidia has done with the GeForce 7950 GX2. It’s one graphics card, but has two printed circuit boards (PCBs), each with a G71 graphics processor. That’s the same one in the GeForce 7900 GTX and GT cards, with 24 pixel-shader units and texture-address units, 8 vertex-shader units, and 16 raster operators. You can find out more about the GeForce 7900 architecture in our initial review of those products.

You can think of the 7950 GX2 as two 7900 GT cards banded together, though the comparison isn’t exact. The core clock speed of the eVGA card we’re reviewing here is 500MHz—up from 450MHz in the 7900 GT. The memory is 600MHz (1200MHz effective), down from 660MHz in the standard GT model. That’s about 10% more core clock speed at 10% less memory bandwidth, which means that in some situations it’ll be a little bit slower and in others a little bit faster, than two 7900 GT cards in SLI mode, depending on the application and it’s demands. The two chips talk to each other through a custom 48-lane PCIe “switch” Nvidia has developed, which provides a 16-lane PCIe interface to the graphics slot and 16-lane PCIe to each of the two GPUs

GeForce 7900 GT cards come with 256MB of graphics RAM, while each of the GPUs on the 7950 GX2 are loaded up with 512MB, for a total of 1GB. While the entire card has 1GB of RAM, all the textures would be repeated on each card, so it’s functionally like having a 512MB graphics card, just with twice the memory bandwidth.

Source: Here

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